The H-bridge is an electronic structure that is generally used to control the polarity at the terminals of a load. It is formed of four switching elements that are generally arranged schematically in the shape of an H. The switches may be relays, transistors, or other switching elements, depending on the targeted application. H-bridges are used in numerous power electronics applications, in particular in controlling motors, converters and choppers, as well as inverters.
As known to those skilled in the art, the H-bridge, as shown schematically in FIG. 1, is used by activating the switches in various combinations in order to achieve the desired connection.
The H-bridge makes it possible to perform two main functions that consist, respectively, in reversing the direction of rotation of the motor M, by reversing the voltage across the terminals of said motor, and in varying the speed of the motor M, by modulating the voltage across the terminals of said motor.
Conventionally, the H-bridge drives a load, typically a DC current electric motor M. The H-bridge is therefore intended mainly to allow said load to be supplied with current in both directions. To this end, the H-bridge consists of an electronic circuit in an H, comprising switches, generally transistors, that are able to be commanded so as to be closed or open, respectively ON or OFF, depending on the direction of the current that is to supply the load. Conventionally, with reference to FIG. 1, those branch parts of the H-bridge that are situated on the ground side, that is to say comprising the second and fourth switches LS0 and LS1, are called ‘low side’; on the other hand, those branch parts of the H-bridge that are situated on the power supply side and comprising the first and third switches HS0 and HS1 are called ‘high side’, in accordance with the technical terminology known to those skilled in the art.
As outlined above, FIG. 1 shows a conventional H-bridge. The transistors HS0, HS1, LS0, LS1 are commanded by a microcontroller (not shown). To supply the motor M with current in a forward direction, the transistors HS0 and LS1 are closed, and the transistors HS1 and LS0 are opened. In this case, the H-bridge supplies power to the motor M in the forward direction, as it is commonly termed, going from the first switch HS0 to the fourth switch LS1 in FIG. 1. By contrast, to supply the motor M with current in the reverse direction, the transistors HS1 and LS0 are closed, and the transistors HS0 and LS1 are opened. In this case, the H-bridge supplies the motor M with a current going in the reverse direction, as it is commonly termed, going from the second switch HS1 to the third switch LS0 in FIG. 1.
The switching operations of the switches are controlled by way of a pulse generator (not shown) having, alternately, a high state and a low state of variable duration.
The first and third switches HS0 and LS0 are moreover connected to the load, formed by the motor M, by a first connection point OUT0, whereas the second and fourth switches HS1 and LS1 are connected to the motor M by a second connection point OUT1.
In addition, the H-bridge may allow magnetic braking to be performed by dissipating the power that is generated. In the general case, the two upper switches HS0, HS1 or lower switches LS0, LS1 are then actuated simultaneously, thereby short-circuiting the terminals of the motor M, and therefore making it unload.
However, there are various strategies for controlling an H-bridge, in particular for the purpose of allowing switching operations that are quicker and emit less electromagnetic interference. Thus, one known technique, referred to using the expression ‘lock anti-phase mode’, makes it possible to control the four switches of an H-bridge using a pulse generator, without having to short-circuit the load in order to unload it, according to the principle briefly described below.
With reference to FIG. 1, the battery BAT supplies the voltage VBAT to the H-bridge. Said H-bridge is controlled using a pulse generator situated at a high level, and the first and fourth switches HS0 and LS1 are closed, so as to load the motor M in the forward direction.
To reduce the current in the load, that is to say to unload the motor M, it is possible to open the first and fourth switches HS0, LS1, a possible battery BAT loading phenomenon then occurring, and then to close the second and third switches HS1, LS0, so as to allow current to flow in the reverse direction, in order to unload the motor M, but taking care not to go as far as reversing the polarity at the load. Such a reversal of polarity in the motor M, if it were to occur, would specifically be liable to damage said motor M, as it would cause an undesired reversal of its direction of rotation.
This requirement to avoid causing a reversal of polarity in the motor M leads to a severe constraint in the controlling of the H-bridge. In practice, according to the prior art, the duty cycle of the pulse generator should be set at 50%, so as to guarantee that unloading the motor M will not result in reversal of the polarity in said motor.
This constraint constitutes a highly inconvenient drawback, and one that is even prohibitive when it is necessary to control an H-bridge driving a DC current electric motor in a motor vehicle.